Japanese Patent Application Publication No. 2010-188825 discloses an electric power steering system configured to apply an assist torque according to a sign of a steering torque inputted to a steering wheel by a driver. However, in this conventional technology, a steering angle neutral position and a steering torque neutral position coincide with each other. For this reason, when a driver performs corrective steering over the steering angle neutral position to return the vehicle running on one side of the lane back to the center of the lane, the sign of the steering torque is inverted and the direction of controlling the force by the driver is switched accordingly. Thus, there has been a problem of a large steering load on the driver.